Slurry for chemical mechanical polishing silicon dioxide

ABSTRACT

A new slurry for shallow trench isolation (STI) processing in the chemical mechanical planarization (CMP) in microelectronic industry comprising an aqueous medium having an abrasive; and a compound which has a carboxylic group and an electrophilic functional group. The combination of ceria and/or titania with amino acids to obtain polishing selectivity&#39;s greater than 5:1. CMP is used for removing the excess oxide and planarizing the substrate and the trench. The silicon nitride acts as a stop layer, preventing the polishing of underlying silicon substrate.

FIELD OF THE INVENTION

The present invention relates to selective slurries used for ChemicalMechanical Polishing (CMP) of Silicon Dioxide, and more specifically toa chemical mechanical polishing composition comprising an abrasive, anaqueous medium and a compound which has a carboxylic acid group and anelectrophilic functional group, for selectively polishing silicondioxide in preference to silicon nitride.

BACKGROUND OF THE INVENTION

There are numerous references within the prior art for the removal ofdielectric films, silicon dioxide, silicon nitride byChemical-Mechanical Polishing (CMP) regulated and moderated by theinteraction of abrasive particulate within a slurry. Changes in theslurry solution properties have a profound effect on the polishingchemistry and relative removal rates of dielectric films. Common slurrysolution properties include pH, temperature, slurry particlecomposition, its size and shape, degree of agglomeration, and slurryweight percent. While the slurry vendor has control over most slurryproperties, the pH and temperature can be controlled during thepolishing process and can have a strong influence on CMP results.

CMP relies on mechanical means with chemical activity to remove andultimately planarize the top film or films on wafers duringsemiconductor processing. The mechanical action during CMP, includingtable speed, applied force, pad hardness, etc., are typically used tocontrol rate, planarity, and uniformity. The chemical portion of CMP isusually exploited to achieve selectivity and help deal with issues ofpattern dependence such as erosion and dishing. Silica slurries aretypically prepared with pH greater than 8, while ceria slurries have pHranging from 4 to 10. In both cases the pH of the suspension can have astrong effect on polish rate and selectivity. Knowing the effect of pHon rate and film selectivity can be used to help understand and controlthe CMP process and achieve the desired result.

A study of the pertinent prior art references indicates that there are afew compositions of CMP slurries which provide a fairly high oxide tonitride removal rate selectivity. Those that are known are summarizedbelow, but none of these prior art references have the composition orthe wide working range of pH.

Conventional CMP slurries used for polishing oxides typically compriseof an abrasive in alkaline medium (high pH). They polish both oxide andnitride at substantial rates with a selectivity of about 10 or below.Hence new slurries which can provide a very high oxide to nitrideselectivity are needed to overcome the problem of polishing nitride andthe underlying silicon substrate.

There are a number of prior art patent references for slurries used inCMP: (1) U.S. Pat. No. 5,738,800 issued in April 1998 to Hosali, et al.;(2) U.S. Pat. No. 5,759,917 issued in June 1998 to Grover et al.; (3) EP0 786 504 A2 published in July 1997 by Kodama et al.; (4) EP 0 846 740A1 published in June 1998 by Ronay; and (5) EP 0 853 335 A2 published inJuly 1998 by Morrison et al.

Additional prior art references in the form of technical publicationsthat are related to the present invention are summarized below.

Chemical Mechanical Planarization of Microelectronic Materials; “8.1.2Shallow Trench Isolation”, by J. M. Steigerwald, S. P. Muraka, and R. J.Gutman; ISBN 0-471-13827-4 (Jon Wiley & Son, Inc.1997), pages 273-274.This reference gives a general discussion of the desirability of usingShallow Trench Isolation in place of Local Oxidation of Silicon (LOCOS).While this reference teaches that conventional CMP processes have lowselectivity between oxides and nitrides, no insight into a potentialcorrection for this problem is taught.

A High Oxide:Nitride Selectivity CMP Slurry for Shallow TrenchIsolation; by Sharath Hosali and Ray Lavoie; in Electrochemical SocietyProceedings, Volume 98-7, pages 218-234. This reference teaches a slurrythat enhances the selectivity rate between silicon oxide and siliconnitride. The slurry taught has cerium oxide as an abrasive with aproprietary solution that inhibits the removal rate of the siliconnitride. The results of this prior art references indicate a high rateof selectivity for unpatterned wafers, however, the selectivity forpatterned wafers is almost the same as with the conventional slurry thatit was compared against.

Application Of Ceria-Based High Selectivity Slurry to STI CMP For Sub0.18 μm CMOS Technologies; by Ki-Sik Choi, Sang-Ick Lee, Chang-II Kim,Chul-Woo Nam, Sam-Dong Kim, and Chung-Tae Kim; CMP-MIC Conference, Feb.11-12, 1999. This paper teaches the use of a ceria based slurry for CMPpurposes in a STI setting. While, the slurry discussed may have beenselective, there is no teaching related to the manufacturing of theslurry. Moreover, in order to prevent dishing from the CMP process,dummy patterns are required. Also, there are some problems related toscratches from the ceria abrasive which require filtering to correct.

A Production-Proven Shallow Trench Isolation (STI) Solution Using NovelCMP Concepts; by Raymond R. Jin, Jeffery David, Bob Abbassi, TomOsterheld, and Fritz Redeker; in CMP-MIC Conference on Feb. 11-12, 1999.This reference discusses the previously discussed problem of having touse dummy patterns to reduce dishing. The solution offered by this priorart reference is to employ a low selectivity, or no selectivity, slurryfor minimizing dishing during over polishing in combination with specialsystem apparatus and polishing heads.

A Wide Margin CMP And Clean Process For Shallow Trench IsolationApplications; by Brad Withers, Eugene Zhao, Rahul Jairath; in CMP-MICConference on Feb. 11-12, 1998, pages 319-327. This reference addressesthe problems of process cost and complexity due to the need for blockmasks, pattern resist etch, high selectivity material overlayers orimplementation of dummy active areas.

It should be readily apparent from the foregoing discussion that thereremains a need within the art for a method of chemical mechanicalpolishing and a slurry for that purpose that is selective and has thewide working range of pH of the present invention.

SUMMARY OF THE INVENTION

The present invention relates to a new slurry for shallow trenchisolation (STI) processing as used in chemical mechanical planarization(CMP) within the microelectronic industry. In particular, the inventionis a CMP slurry comprising of an abrasive, an aqueous medium and acompound which has a carboxylic group and an electrophilic functionalgroup.

In the manufacturing of integrated circuits, the various devices in alayer must be isolated from one another. This can be accomplished bySTI, among other methods. Compared to other methods, STI has theadvantage of providing higher packing density for the devices. In theSTI process, silicon dioxide is used as the isolating material. A layerof silicon nitride is deposited on silicon and a shallow trench isetched into the substrate using photolithography masks. Silicon dioxideis then deposited into the trench and over the nitride layer. The excessoxide on the top of nitride must be removed and the trench planarized inorder to prepare for the next step which is usually the growth of gateoxide and deposition of poly silicon gate.

CMP is used for removing the excess oxide and planarizing the substrateand the trench. The silicon nitride acts as a stop layer, preventing thepolishing of underlying silicon substrate. In order to achieve adequateplanarization and minimal overpolishing, the slurry used for CMP musthave a high oxide to nitride removal rate ratio. This and otherobjectives are satisfied by the present invention in providing a slurryfor chemical mechanical polishing comprising an aqueous medium withabrasive particles, and a compound which has a carboxylic acid group andan electrophilic functional group, for selectively polishing silicondioxide in preference to silicon nitride; and applying the slurry at apolishing interface between a polishing pad and the composite comprisedof silica and silicon nitride.

ADVANTAGES OVER PRIOR ART

The slurry provides high oxide:nitride removal rate selectivity over awide range of pH (at least between pH of 6 and pH of 11). It is notnecessary to have a surfactant in the slurry to obtain the highselectivity in the present invention. It provides a wide operating rangegiving numerous opportunities to optimize other parameters (like surfacequality) without the loss of selectivity. Many of the compounds that areused as additives are environmentally benign thus minimizing the cost ofwaste water treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the oxide and nitride removal rates of alow selectivity slurry;

FIG. 2 is a graph illustrating oxide:nitride selectivity of a lowselectivity slurry;

FIG. 3 is a graph illustrating the oxide and nitride removal rates of ahigh selectivity slurry; and

FIG. 4 is a graph illustrating oxide:nitride selectivity of a highselectivity slurry.

DETAILED DESCRIPTION OF THE INVENTION

It has been discovered that a slurry for shallow trench isolation (STI)processing that can be used for chemical mechanical planarization (CMP)in the microelectronic industry to solve the prior art problem ofpolishing nitride and the underlying silicon substrate and yielding veryhigh oxide to nitride selectivity. In particular, the invention is a CMPslurry comprising of an abrasive, an aqueous medium and a compound whichhas a carboxylic group and an electrophilic functional group.

In the manufacturing of integrated circuits, the various devices in alayer must be isolated from one another. This can be accomplished by anSTI process, wherein silicon dioxide is used as the isolating material.A layer of silicon nitride is deposited on silicon and a shallow trenchis etched into the substrate using photolithography masks. Silicondioxide is then deposited into the trench and over the nitride layer.The excess oxide on the top of nitride must be removed and the trenchplanarized in order to prepare for the next step which is usually thegrowth of gate oxide and deposition of poly silicon gate.

FIGS. 1 and 2 illustrate the results of a low selectivity slurrycomprising 1% (by wt) ceria in water. The pH is adjusted with 40% (bywt) KOH. The polishing conditions are 5.6 psi down pressure, 30 rpmtable speed, 30 rpm quill speed, 25° C. temperature and 240 cc/minslurry flow rate.

FIGS. 3 and 4 illustrate the results of a high selectivity slurrycomprising 1% (by wt) ceria and 2% (by wt) L-Proline (additive) inwater. The pH is adjusted with 40% (by wt) KOH or 1 N Nitric acid. Thepolishing conditions are 6 psi down pressure, 2 psi back pressure, 40rpm table speed, 40 rpm quill speed, 20° C. temperature and 340 cc/minslurry flow rate. The results shown in FIGS. 3 and 4 clearly illustratea dramatic improvement in selectivity as compared to that as shown inFIGS. 1 and 2.

It is clear that the presence of the additive, L-Proline, reduces thepolish rate of nitride dramatically, yielding a very high selectivity ina wide range of pH.

Other additive with carboxylic groups and electrophilic functional groupalso were tested and the results are given in table 1. The slurries had5% (by wt) ceria, given amount of the additive and the pH was adjustedto 10 with 40% (by wt) KOH or 30% Ammonia solution. Polishing conditionswere 5.6 psi down pressure, 30 rpm table speed, 30 rpm quill speed, 25°C. and 240 cc/min slurry flow rate.

TABLE 1 Additive concentration (by Oxide polish Nitride polish wt %)rate (nm/min) rate (nm/min) Selectivity 0 489 66 <8 1% glycine 547 34 161% L-alanine 545 20 27 1% L-Proline 487 12 41 4% L-Proline 430 2 230

Titania particles were used as abrasive in the following slurry. Theslurry had 5% (by wt) titania, given wt % of L-Proline and the pH wasadjusted to 10 with 30% ammonia solution. The polishing conditions were5.6 psi down pressure, 40 rpm table speed, 40 rpm quill speed, 25 C and240 cc/min slurry flow rate. The results are shown in table 2.

TABLE 2 Additive concentration (by Oxide polish Nitride polish wt %)rate (nm/min) rate (nm/min) Selectivity 0 57 36 <2 4% L-Proline 73 1 73

Thus it is clear that while the presence of the additive is crucial toreduce the polish rate of the nitride, the additive is effectiveirrespective of the type of abrasive.

The slurry provides high oxide:nitride removal rate selectivity over awide range of pH (at least between pH of 6 and pH of 11). Unlike U.S.Pat. No. 5,738,800, it is not necessary to have a surfactant in theslurry to obtain the high selectivity in the present invention. It isbelieved that the wide operating range will provide a lot of room tooptimize other parameters (like surface quality) without the loss ofselectivity. Many of the compounds that are used as additives areenvironmentally benign thus minimizing the cost of waste watertreatment.

Permissible alternatives in the elements, materials, steps,arrangements, of the present invention including the broadestpermissible range for conditions, and proportions follow. The additivehas a carboxylic group and another functional group which iselectophilic (i.e. attracts electrons). Examples of electrophilic groupsinclude, but are not limited to, amine groups, chloride, hydroxyl groupand carboxylic group. If the electrophilic group is in the alphaposition (close to the carboxylic group) it is especially effective.Alpha amino acids are the preferred additives since they are costeffective and environmentally friendly.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A method of polishing a composite comprised ofsilicon dioxide and silicon nitride comprising the steps of: providing aslurry comprising an aqueous medium, abrasive particles, and a compoundwhich has a carboxylic group and an electrophilic functional group, forselectively polishing silicon dioxide in preference to silicon nitride;and applying the slurry at a polishing interface between a polishing padand the composite comprised of silica and silicon nitride. 2.The methodof claim 1 wherein the slurry has a pH of between 6 to
 12. 3. The methodof claim 1 wherein the electrophilic functional group is not a hydroxylgroup.
 4. The method of claim 1 wherein the compound further comprisesone of the following: proline; glycine; or alanine.
 5. The method ofclaim 1 wherein the electrophilic functional group does not have adissociable proton.
 6. The method of claim 1 wherein the abrasiveparticles are ceria.
 7. The method of claim 6 wherein the ceria isprepared to have an isoelectric point of about 9.5.
 8. The method ofclaim 1 wherein the slurry further comprises KOH.
 9. The method of claim1 wherein the slurry further comprises water, ceria with an isoelectricpoint of about 9.5, and a pH between 7-10.
 10. The method of claim 1wherein the abrasive further comprises titania.